Pfc

[johnboy6083]

hi ladies and gents,

system= TNS, tranny 5m away on pole, 70mm tails, 3ph

tester= megger 1552

was on a shutdown the other night, and after doing my bit in the main MCC, i did my live tests. i disconnected all bonding and earthing conductors, and took my readings. ZS is 0.01, which i expected and PEFC measured at > 20ka (calculated at 24.5 kA). same reading for PSCC (calculated at 24.5 x 2 = 49kA).

from this MCC, i installed a 63A switchfuse fed by 25mm2 tails from busbar, which feeds a 25m run 25mm2 SWA to the VSD panel for 2 pumps.

When testing Zs for the VSD panel i got 0.02.
PEFC was 7.2Ka and as the megger allows testing between phases, i measured that too, but got 5.5kA?

why was this? all phases tested frine on R1+R2 same readings (0.05) and Zs was the same on all too, (0.02).

I was taught that on a TNS, PSCC on a 3ph system will always be higher than PEFC?

did i even need to test PSCC and PEFC at the VSD panel? i did it anyway. would the fuses (63A, bs 88-2) discriminate at those currents, or would it take out the 200A main fuses in the main MCC also?

thanks in advance for your advice



John

 

[Chr!s]

was on a shutdown the other night, and after doing my bit in the main MCC, i did my live tests. i disconnected all bonding and earthing conductors, and took my readings. ZS is 0.01, which i expected and PEFC measured at > 20ka (calculated at 24.5 kA). same reading for PSCC (calculated at 24.5 x 2 = 49kA).

MCC ?, Is .01 Ze, have you doubled your L - E value to produce your PSCC?, you need to double your phase(L - N) voltage for this.

from this MCC, i installed a 63A switchfuse fed by 25mm2 tails from busbar, which feeds a 25m run 25mm2 SWA to the VSD panel for 2 pumps.

When testing Zs for the VSD panel i got 0.02.
PEFC was 7.2Ka and as the megger allows testing between phases, i measured that too, but got 5.5kA?

why was this? all phases tested frine on R1+R2 same readings (0.05) and Zs was the same on all too, (0.02).

Was the CPC provided by a core as well as the armoring ?

did i even need to test PSCC and PEFC at the VSD panel? i did it anyway. would the fuses (63A, bs 88-2) discriminate at those currents, or would it take out the 200A main fuses in the main MCC also?

Yes, its not so much about discrimination, its more about whether the installation can safely interrupt the expected pfc at the various points.

I was taught that on a TNS, PSCC on a 3ph system will always be higher than PEFC?

Are you 100% its tns and not tncs?

 

[johnboy6083]

it was def a TNS mate, i could see the supply cable at both ends, as the tranny was on a pole, less than 5m away from the main head. PEFC and PSCC were exactly the same at the intake point, although i reckon that was due to the tester not having a high enough accuracy to detremine any difference at such a low reading.

MCC is motor control centre, in a water treatment plant.

The armour for the submain to the VSD panel was bonded at both ends, and a seperate 16mm earth installed also.

 

[malcolmsanford]

Was you using the armoured as a CPC combined with the 16mm, becasue was wondering if it was a multi-core why you bonded it at both ends mate.

 

[johnboy6083]

steel gland plates at both ends, with flying leads from the plates. both ends had plates manfactured like this.

I wasnt responsible for design mate, just installing.

Also they insist that both ends of power cables are bonded. This includes e stops!

 

[Guest 004]

If Ze = 0.01 then PFC = 23Ka single phase, then just multiply by root 3 (1.732) = 39.83 Ka, which is very close to breaking capacities of some types of MCB/MCCB's, but as you have a BS88, the breaking capacity is 80Ka, no problems.

 

[johnboy6083]

why did the meggar measure only 5.5Ka for PSCC , and PEFC at 7.2kA a the VSD end? why do they teach you to mulitply PSCC Live to neutral x 2, when my readings dont reflect that?

measured voltage on the system was 245v mate, hence 24kA.

 

[Guest 004]

why did the meggar measure only 5.5Ka for PSCC , and PEFC at 7.2kA a the VSD end? why do they teach you to mulitply PSCC Live to neutral x 2, when my readings dont reflect that?

measured voltage on the system was 245v mate, hence 24kA.

I was taught to use root 3 as this is the correct way, all the lazy peeps say double it, because they cannot be bothered to calculate. Dont know why your meter is giving those readings, but BY using math we know what the answer is!
If you want to record your 'measured' reading (bearing in mind you could be doing something wrong) I would suggest that you make a comment on the test sheet. Just cover your ---

 

[johnboy6083]

i think i will record the calculated answer, then I cant go wrong with that. i was wondering if there was an explanation to why the tester gave that reading.

 

[Chr!s]

why did the meggar measure only 5.5Ka for PSCC , and PEFC at 7.2kA a the VSD end? why do they teach you to mulitply PSCC Live to neutral x 2, when my readings dont reflect that?

measured voltage on the system was 245v mate, hence 24kA.

you would normally not take the neutral into the equation.

If you tested between L - N you may have a difference due to the resistance of the neutral conductor. Some supply cables come with a smaller neutral to that of the line

 

[Chr!s]

If Ze = 0.01 then PFC = 23Ka single phase, then just multiply by root 3 (1.732) = 39.83 Ka, which is very close to breaking capacities of some types of MCB/MCCB's, but as you have a BS88, the breaking capacity is 80Ka, no problems.

Not necessarily, it depends upon the fault levels, the energy let through values of the upstream protective device/s and the breaking capacity of the downstream devices.

 

[johnboy6083]

chris, how did you get to the 7.6 Ka in your earlier post?

 

[Guest 004]

Not necessarily, it depends upon the fault levels, the energy let through values of the upstream protective device/s and the breaking capacity of the downstream devices.

Can you elaborate more please m8? upstream/downstream?

Would also like to know how you got to your values of 7.6

 

[Chr!s]

Theres two calcs to consider

L - L

U/Zs + 2R1

Zs = line to line impedance, R1 Line resistance of 1 line

Three Phase

U/1.73 divided by Zs + R1, Zs now the line to neutral impedance.

 

[Chr!s]

Can you elaborate more please m8? upstream/downstream?

Take the BS 88 as your up stream, Breaking cap 80 Ka

now if you have a high pfc(50Ka) let say at the terminals of a 15ka rated Mccb, the let through energy of the bs 88 may be say 20 ka, so the mcb needs to be of a higher rating.

The pfc need to be calculated at each point back up protection may be provided by an upstream device, though this would need to be calculated and not assumed.

So down stream devices need to be able to break the 20 ka